Odd monitor behavior with Win 10

I have a dual monitor setup. When I power on the machine both monitors show the mobo splash screen then the Windows logo, but when it goes to the login screen it's as if it loses the signal like the computer was turned off. I can still login, blindly, since the main side is blank. The second monitor then displays as normal. Since I can see the task bar on it I can restart. When it retarts both monitors work normally.
 
The only way I can get it working right every time is to go through that whole thing of booting and rebooting. It just started doing that about a week ago. I've made no changes to the system by adding or removing anything before it started. Just odd.
 
Anyone else here ever have this happen or have any clue what could be causing it?

I had this happen several times ages ago, Windows 7 I think.

I'm not sure what I did to fix it, but I think I shut down, disconnected the secondary screen - the one I didn't want Windows defaulting to on boot/waking - then booted and reconnected the other screen after Windows had loaded then checked which screen it thought should be the primary one.

I've a feeling that the problem has something to do with which screen had focus the last time Windows was shut down or put to sleep, it then recalling that screen as the one to activate first. Though that's a guess on my part.